<p>Large-scale disaster response heightens the need for both vertical and horizontal collaboration, yet the mechanisms that shape these networks—and their evolution under institutional change—remain insufficiently understood. This study integrates theories of collaboration risk, resource dependency, and attribute-based homophily to construct an analytical framework and test hypotheses. Using data from two floods in 2016 and 2020, we examine the horizontal and vertical network formation in China before and after the establishment of the Ministry of Emergency Management in 2018. Results show that both networks favor bonding ties to share risk; vertical networks exhibit stronger resource-based, cross-sector linkages with public actors, and institutional reform significantly increases cross-sector and resource-based collaboration within horizontal networks in China. This study proposes fostering closed trust structures, empowering emergency agencies as resource hubs, and institutionalizing coordinating bodies with clear mandates and funding to integrate vertical and horizontal capacities for more resilient disaster governance.</p>